NAME

SYNOPSIS

DESCRIPTION

swapon
is used to specify devices on which paging and swapping are to take place.

The device or file used is given by the
specialfile
parameter. It may be of the form
-L label
or
-U uuid
to indicate a device by label or uuid.

Calls to
swapon
normally occur in the system boot scripts making all swap devices available, so
that the paging and swapping activity is interleaved across several devices and
files.

swapoff
disables swapping on the specified devices and files.
When the
-a
flag is given, swapping is disabled on all known swap devices and files
(as found in
/proc/swaps
or
/etc/fstab).

OPTIONS

-a, --all

All devices marked as ``swap'' in
/etc/fstab
are made available, except for those with the ``noauto'' option.
Devices that are already being used as swap are silently skipped.

-d, --discard[=policy]

Enable swap discards, if the swap backing device supports the discard or
trim operation. This may improve performance on some Solid State Devices,
but often it does not. The option allows one to select between two
available swap discard policies:
--discard=once
to perform a single-time discard operation for the whole swap area at swapon;
or
--discard=pages
to discard freed swap pages before they are reused, while swapping.
If no policy is selected, the default behavior is to enable both discard types.
The
/etc/fstab
mount options
discard,
discard=once,
or
discard=pages
may also be used to enable discard flags.

-e, --ifexists

Silently skip devices that do not exist.
The
/etc/fstab
mount option
nofail
may also be used to skip non-existing device.

-f, --fixpgsz

Reinitialize (exec mkswap) the swap space if its page size does not
match that of the current running kernel.
mkswap(2)
initializes the whole device and does not check for bad blocks.

-h, --help

Display help text and exit.

-L label

Use the partition that has the specified
label.
(For this, access to
/proc/partitions
is needed.)

-o, --options opts

Specify swap options by an fstab-compatible comma-separated string.
For example:

swapon -o pri=1,discard=pages,nofail /dev/sda2

The opts string is evaluated last and overrides all other
options.

-p, --priority priority

Specify the priority of the swap device.
priority
is a value between -1 and 32767. Higher numbers indicate
higher priority. See
swapon(2)
for a full description of swap priorities. Add
pri=value
to the option field of
/etc/fstab
for use with
swapon -a.
When no priority is defined, it defaults to -1.

-s, --summary

Display swap usage summary by device. Equivalent to "cat /proc/swaps".
Not available before Linux 2.1.25. This output format is DEPRECATED in favour
of --show that provides better control on output data.

--show[=column...]

Display a definable table of swap areas. See the
--help
output for a list of available columns.

--noheadings

Do not print headings when displaying
--show
output.

--raw

Display
--show
output without aligning table columns.

--bytes

Display swap size in bytes in
--show
output instead of in user-friendly units.

-U uuid

Use the partition that has the specified
uuid.

-v, --verbose

Be verbose.

-V, --version

Display version information and exit.

NOTES

You should not use swapon on a file with holes.
This can be seen in the system log as

swapon: swapfile has holes.

The swap file implementation in the kernel expects to be able to write to the
file directly, without the assistance of the filesystem. This is a problem on
preallocated files (e.g.
fallocate(1))
on filesystems like XFS or ext4, and on copy-on-write
filesystems like btrfs.

It is recommended to use
dd(1)
and
/dev/zero
to avoid holes on XFS and ext4.

swapon
may not work correctly when using a swap file with some versions of
btrfs. This is due to btrfs being a copy-on-write filesystem: the
file location may not be static and corruption can result. Btrfs actively
disallows the use of swap files on its filesystems by refusing to map the file.

One possible workaround is to map the swap
file to a loopback device. This will allow the filesystem to determine the
mapping properly but may come with a performance impact.

Swap over NFS may not work.

swapon
automatically detects and rewrites a swap space signature with old software
suspend data (e.g S1SUSPEND, S2SUSPEND, ...). The problem is that if we don't
do it, then we get data corruption the next time an attempt at unsuspending is
made.